Refractive Index and the Lorentz–Lorenz Function for Gaseous and Liquid Argon, Including a Study of the Coexistence Curve near the Critical State

Measurements of the refractive index of gaseous and liquid argon at 5893 Å were carried out along the coexistence curve and along eight isotherms from 133° to 173°K and for pressures from 15 to 100 atm. The measurements were combined with existing density data in the literature in order to calculate values of the Lorentz–Lorenz function. The experiments included detailed studies of the critical region, and the observed values of the refractive index were used to define the shape of the gas–liquid coexistence curve near the critical state. The critical temperature is estimated to be 150.704 ± 0.015°K. Values of nc = 1.0859, Pc = 48.18 atm, and LLc = 4.200 cm3 / mole were also determined. The shape of the coexistence curve is often described in terms of a critical coefficient β, with ρl–ρg = A0′(Tc − T)β. By detailed statistical analysis of the refractive index data, including variation of Tc over a slight range, β was found to be 0.364 ± 0.007. This is within the uncertainty range of previous determinations, but seems clearly higher than the value of 0.313 ± 0.004 predicted for the three-dimensional lattice gas. Measurements of the refractive index on the coexistence curve outside the critical region yielded an average value of the Lorentz–Lorenz function of 4.152 cm3/mole for the gas; the average liquid value is 4.213 cm3/mole. The values of the Lorentz–Lorenz function on eight isotherms between 133° and 173°K show a peak near the critical density, but within the experimental uncertainty; except for such peaks, the Lorentz–Lorenz values along the isotherms lie within the range 4.16–4.26 cm3/mole. All available values for the Lorentz–Lorenz function for argon ranging through three states of matter and temperatures from 20° to 300°K fall within ±1.5% of 4.19 cm3/mole.